[Neural mechanisms of syntax]

Agrammatism is defined as a disorder of language expression, characterized by the omission of the relational words: articles, prepositions, conjunctions, and minor modifiers. The resulting output is generally referred to as nonfluent aphasia. Disorder of syntactic comprehension is also observed and we have implemented a binary decision task for a clinical investigation. However, this task could not make clear how and why a patient made errors. Hence we used here the object manipulation task, which patients manipulated some animal toys to depict the sentence they heard, for patients with infarcts in BA44, 45/46. Stimulus sentences contained several sentence patterns, including scrambled sentences grammatically possible in Japanese but ungrammatical in English. Results were as follows: i) All patients could not understand scrambled active sentences. Additionally, complex sentences for BA44, and three-place verb sentences for BA45/46, were difficult to understand. ii) Unlike English/French patients, Japanese patients could depict passive sentences easily rather than three-place verb sentences (BA45/46). iii) All patients made common errors: selecting another item; default strategy; simplifying sentence construction. Considering these results and recent studies, we speculate that syntax reflects rules to segmentalize (or categorize) the external world to process sensorimotor information effectively for thinking and communication.